It is known to use polyisocyanates and blocked polyisocyanates as crosslinkers to cure active hydrogen-containing compounds. Among these can be mentioned 1,6-hexamethylene diisocyanate and 2,4-tolylenediisocyanate, and the like. The isocyanate groups react with the active hydrogen compound to form urethane linkages, urea lingages, and the like. These isocyanates are frequently used and sold in the so-called blocked form, in which they have been pre-reacted, e.g., with phenol, or with another alcohol, or a beta diol, to form urethane compounds. When the curable compositions are prepared, they are generally stable at moderate temperature, but after heating, they will de-block by yielding the polyisocyanate and phenol or the respective alcohol, and then cure by reaction with the active hydrogen compound. In many cases the isocyanate equivalent weight of the crosslinker is relatively high because the compounds are difunctional and the blocking agents themselves have relatively high molecular weights. Moreover, aromatic polyisocyanates tend to form coatings which discolor on exposure to light. Aliphatic polyisocyanates on the other hand have superior resistance to degradation by light, but they are less reactive than aromatic isocyanates.
Mono- and diisocyanato-s-triazines have been reported (Ubrich van Gizycki, Angew Chem. Internat. Edit., Vol. 10 (1971, No. 6), having structures as follows: ##STR1## where R=Cl, F or CCl.sub.3 and ##STR2## where R=Cl, CCL.sub.3 or C.sub.6 H.sub.5. These compounds were prepared by reaction of the appropriate amino-s-triazine (0.1 mol.) with oxalyl chloride (0.3 to 0.5 mol per amino group) in anhydrous toluene or chlorobenzene (1-2 mol) or thiophane dioxide where R=F, the mixture being boiled at reflux until gas evolution is complete (7-8 hours). However, triisocyanato triazines have not been heretofore reported, to Applicants' knowledge.
It has now been discovered that melamine triisocyanate and derivatives thereof containing blocked isocyanate groups comprise a family of crosslinking agents with highly desirable properties. These compounds provide the light stability and superior environmental resistance of aliphatic isocyanates and their derivatives while at the same time retaining the superior reactivity and versatility of aromatic isocyanates and their derivatives. Accordingly, the compounds of the instant invention are seen to provide the combined advantages of both the aliphatic and aromatic isocyanates without the disadvantages heretofore associated with either. The triisocyanate derived from melamine, being tri-NCO functional and of relatively very low NCO equivalent weight (68) is an excellent crosslinker, producing urethane, urea, and other functionally cross-linked thermosetting systems. It is also a crosslinker for low temperature (room to slightly elevated temperature) curing thermosets, in the form of coatings, reactive injection moldings (RIMs), composites, adhesives, binders, etc., in its unblocked state. When blocked by reaction with alcohol, phenols, or other blocking agents, the new compounds are useful in one-component, heat cured systems.